Microstructural studies of the copper promoted iron oxide/chromia water-gas shift catalyst

The microstructure of an iron oxide based high temperature water-gas shift (WGS) catalyst has been studied by X-ray diffraction (XRD), high resolution electron microscopy (HREM), high spatial resolution energy dispersive X-ray analysis (STEM-EDX) and X-ray photoelectron spectroscopy (XPS). The iron oxide contains a chromia additive that structurally stabilises the catalyst. Chemical microanalysis indicates that the chromia forms a solid solution within the magnetite Fe3O4 lattice and that no discrete chromia phases are formed. The level of Cr dissolution varies greatly at the intergranular level. XPS and STEM-EDX studies suggest that the activated catalyst is surface enriched in chromia. On the basis of these results a model for the stabilising effect is proposed in which the enriched surface 'shell' encapsulates each catalyst grain and, being more thermodynamically stable than the iron-rich core, reduces ion diffusion and sintering effects. A Cu dopant is often added to the Fe3O4/Cr2O3 catalyst to promote its activity. Chemical microanalysis shows that this dopant also exists in solid solution. Furthermore, STEM-EDX and XPS analyses indicate that the catalyst grains also exhibit a strong surface enrichment in the Cu species. Analysis of aged ex-reactor specimens suggest that deactivation occurs as a result of eventual sintering, with a substantial increase in grain-size relative to the fresh catalyst. The Cu dopant is also demonstrated to have a propensity to segregate as CuO on the surface of the catalyst grains in aged specimens.